Effect of presynaptic inhibition on axonal potentials, terminal potentials, focal synaptic potentials, and EPSPs in cat spinal cord

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Abstract
In the L7 spinal cord of barbiturate-anesthetized cats, the effects of group I conditioning volleys in the posterior biceps-semitendinosus nerve on potentials produced by test volleys in the medial gastrocnemius nerve were studied. Conditioning stimulation decreased the latency of the positive peak of the terminal potential and decreased the amplitude of action potentials (AP), terminal potentials (TP), focal synaptic potentials (FSP) and excitatory postsynaptic potentials (EPSP) produced by test stimuli. The decreased latency and amplitude of these potentials were maximal at a 20 ms conditioning-testing interval, returning to control values at intervals of 200 ms or more. The reduction in TP latency and depression in TP, FSP and EPSP amplitude occurred in triceps surae motoneuron pool and intermediate nucleus, with the maximal changes observed in the motoneuron pool. In triceps surae motoneuron pool, the time course of FSP and EPSP depression following the conditioning volleys paralleled the time course of TP depression with test intervals > 30 ms. With test intervals of 30 ms or less, depression of FSP and EPSP consistently exceeded that of TP, presumably due to an early and brief increase in motoneuron conductance. This early conductance change, having a total duration of < 50 ms, was attributed to postsynaptic inhibition from the conditioning stimulation. Conditioning volleys confined to group I afferents lead to primary afferent depolarization of group I afferent terminal aborizations, which in turn depresses synaptic transmission (presynaptic inhibition).

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